It is known in the art that in order to get acceptable adhesion of a coating to a surface, the surface must typically be pretreated in some manner. The pretreatment can take the form of chemical treatment or mechanical treatment, or a combination thereof. The primary purpose of the pretreatment is to remove surface impurities or contaminants which can either prevent the coating from adhering, or can result in a corrosive substance being locked-in between the interface of the coating and the surface.
Chemical treatments which have been used to prepare surfaces for coatings include solvents such as trichloroethane, caustics, high purity distilled or deionized water, soaps, detergents, acids, etc. In the automobile industry, for example, it is common to pretreat metal surfaces with a phosphatizing composition prior to coating.
It is also common in preparing metal surfaces for primers and/or protective coatings to mechanically abrade surfaces to remove oxidized metal, surface impurities, and contaminants.
An example of a mechanical abrasive treatment is sand blasting. This is typically done by entraining an abrasive material such as slag or sand in a high velocity air stream to impart sufficient energy to the particles such that upon impact they will abrade the surface of the metal and remove the surface layer of oxidized metal, surface impurities, or contaminants. The abrasive particles are, typically, continuously recycled. In such an abrasive process, it is important to paint the bare metal surface as soon as possible after abrasion to prevent a new oxidized layer from forming at the surface and to minimize other surface contaminants.
U.S. Pat. No. 2,703,550 discloses a process for removing scale deposits from wire. The wire is passed through a rotating steel drum containing steel balls which rub the wire clean of scale deposits. U.S. Pat. No 4,244,989 discloses a method of abrading and rust proofing by using dry abrasive particles coated with a rust proofing composition such as a resin binder. U.S. Pat. No. 3,498,768 discloses a method of abrasive blasting and applying a rust inhibitor. The process uses dry fatty acid coated abrasive particles as a rust inhibitor. The blasted surface is prevented from oxidizing by the rust inhibitor.
U.S. Pat. No. 4,091,125 discloses a method of roughening copper foil in the manufacture of printed circuit boards by using a mechanical abrading process. A pressurized air and grit mixture is sprayed from a nozzle onto a copper foil so that the abraded surface will adhere to a resin impregnated substrate.
Japanese Patent No. 57-126972 relates to a method of matting a surface by applying a paint containing a matting agent.
As previously mentioned, there are several reasons why it is necessary to pretreat a surface prior to applying a coating. First of all, there may be residual contaminants on the surface, as the result of manufacturing processes or oxidation, which must be removed in order for the coating to adhere. Secondly, it may be necessary to remove corrosive contaminants so that they will not be locked in by the application of the coating. Finally, the surface may be inherently slippery, or slippery due to residual contaminants, and it may be necessary to roughen or abrade the surface in order for the coating to properly adhere.
In the electronics industry, it is frequently necessary to apply conformal coatings to assembled circuit boards. An assembled printed circuit board (circuit board assembly) consists of various types of electrical and electronic components soldered into a printed circuit board. The conformal coatings are typically resinous compositions such as urethanes, epoxys, polyesters, alykds, acrylics, etc. which form a film over the printed circuit board and components in order to provide a protective barrier against corrosion, humidity, and any atmospheric contaminants. The conformal coating also performs an electrical insulating function. The conformal coatings are typically required by military and government specifications. It is known in the art that during the course of printed circuit board assembly, numerous contaminants accumulate as residues on surfaces of the printed circuit board and the electrical and electronic components. The contaminants can also be absorbed into the interior of a printed circuit board. These contaminants include silicone oils, adhesives containing silicone, RTV compounds, finger prints, particulate contaminants, cleaning solution residues, etc. It is known in the art that the typically used conformal coatings do not adhere satisfactorily to surfaces when these contaminants are present. The conformal coatings tend not to wet the contaminated surfaces properly, resulting in a nonuniform distribution of the coating across the various surfaces of the printed circuit board and the electronic components. This results in areas of the board assembly where the coating is not present at all, e.g. pin holes, resulting in the impairment of the integrity of the coating, in that pathways for humidity, contaminants, electrical leakage etc. are present in the coating. The problem is especially pronounced when using automatic conformal coating equipment as evidenced by extremely high quality control rejection rates in the range of 98 %. A printed circuit board assembly having a defective conformal coating must be reprocessed by typically first dipping the board in a hot lye bath to strip the coating and then attempting to reapply the conformal coating. The additional stress of subjecting the board and components to this rework procedure can have detrimental effects on the components and the board. Typically, at least ten percent of the reworked printed circuit board assemblies must be scrapped because of damage sustained during rework.
It is presently known in the art to pretreat printed circuit board assemblies prior to conformal coating operations. This pretreatment typically takes the form of first placing the assemblies in a high purity dionized water bath, next placing the assemblies in a solvent bath such as 1,1,1 -trichloroethane or Freon.TM. solvent and then dipping in an alcohol bath. However, these pretreatments do not have the ability to sufficiently prepare the surface of the printed circuit board assemblies so that a conformal coating can be applied having satisfactory adherence and absolute integrity.
Presently, in order to compensate for the surface contaminants which remain after pretreating in high purity deionized water, organic solvent, and alcohol, it is necessary to apply the conformal coating by hand with a spray gun to achieve an acceptable coating having integrity. Since the contaminants cause the coating to bead-up rather than flow and adhere to the surfaces, the operator compensates for this phenomenon by applying increased amounts of the coating in those areas of the board having inadequate adhesion, thereby producing a somewhat thicker coating. Although the pretreatment removes silicone and other contaminants from hard surfaces, it has been observed that silicone and similar contaminants are absorbed by printed circuit boards. The printed circuit board is typically a glass fiber-epoxy/phenolic composite; pretreatment does not remove internally absorbed contaminants in these boards which tend to bleed to the surface. The problem present with hand application is that although the coating does have integrity, it is a thicker coating than that which is desired. It is known in the art that the useful life of a coating is related to an optimum thickness. An overly thick coating is undesirable. Also, the coating never actually adheres to the contaminated areas, but the coating forms bridges over those areas containing contaminants. There are several problems associated with this approach. As previously mentioned, it is known that overly thick coatings tend not to adhere to a surface over time. In addition, this method is labor intensive. An automated system is preferred. Finally, even though a coating with integrity is applied by this method, the adhesion to the surface is less than adequate due to the previously mentioned bridging phenomena. This lack of complete adhesion will eventually result in a deterioration of the protective barrier.
It is known in the art to chemically clean and/or wash printed circuit board assemblies prior to applying a coating, however, due to the nature and quantity of contaminants typically present on an assembled printed circuit board, this approach has not produced satisfactory results, especially with automatic coating equipment. Chemically cleaning and/or washing alone is inadequate. The types of chemicals which can be used are limited by the materials used to construct electronic components and printed circuit boards. Attempts at abrading printed circuit board assemblies prior to coating have been similarly unsuccessful. The time delay between the abrasion of the surface and the application of the conformal coating has been observed to result in a bleeding of the contaminants, particularly silicones, from the inner part of the board to the outer surfaces resulting in the previously mentioned problems.
Accordingly, what is needed in this art is an improved method of preparing a surface to receive a coating.